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An element's atomic number is the same as the number of protons in the element. Because potassium has an atomic number of 19, it has 19 protons.

A neutral atom has the same number of protons and electrons, so one neutral atom of potassium must also have 19 electrons.

The number of neutrons in an element is equal to the element's mass number minus the number of protons in one atom of the element, according to the Thomas Jefferson National Accelerator Facility Office of Science Education. To find the mass number, round the atomic weight of the element that's found below the element's symbol on the periodic table to the nearest whole number. Potassium has an atomic weight of 39.0983, so its mass number is 39. There are 19 protons, so there must be 20 neutrons, which is the mass number of 39 minus 19 protons.